This could in theory allow importing subsets of files with less memory
use. Rather than building up a big import list and then filtering it to
a smaller list of wanted files, support optionally filtering wanted
files first.
So far, the directory special remote implements it and will probably use
less memory. (Since dirContentsRecursiveSkipping does lazy streaming.)
Implementation in Remote.S3 is incomplete and fails to compile. Bit of a
mess with ResourceT needing to use Annex.
Also, in Remote.S3, filtering is not done for old versions.
And mkImportableContentsUnversioned is doing now redundant work
to filterwanted.
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